To replace stair spindles, first remove the filet next to the spindle to be removed and tap the spindle to remove it from the filet. Use the old spindle to create a new one, insert it into the top channel, position it upright and replace the filet.
To remove the filet or the horizontal piece of wood between spindles at the lower channel, insert a 3/4 inch screw around 3/16 inches into the filet one inch away from the spindle to be replaced. Pull the screw with a pair of pliers to make the filet come out of the channel.
To remove the spindle from the filet, tap its bottom with your hands towards the filet's open end. When the spindle loosens, allow it to swing out and come out of the square hole in the top rail channel.
To create a new spindle of the same size as the old one, measure the latter and use this measurement to trim the replacement spindle with a miter saw. Sand the edges with sandpaper.
To install the new spindle, insert its top into the square hole of the top rail channel. If the fitting is too tight, sand the bottom side of the replacement spindle. Then tap the spindle's bottom to straighten it into position. Take out the screw from the filet, place it upside down in the bottom channel, sand the surface, and tap it into position. Drive a 1-inch finish nail using a hammer into the filet.